Photovoltaics (PV) or solar cells are devices which convert sunlight into direct current (DC) electrical power. A typical PV cell includes a p-type silicon wafer, substrate, or sheet typically less than about 0.3 mm thick with a thin layer of an n-type silicon material disposed on top of the p-type substrate. The generated voltage, or photo-voltage, and generated current by the photovoltaic device are dependent on the material properties of the p-n junction, the interfacial properties between deposited layers, and the surface area of the device. When exposed to sunlight (consisting of energy from photons), the p-n junction of the PV cell generates pairs of free electrons and holes. The electric field formed across the depletion region of the p-n junction separates the free electrons and holes, creating a voltage. A circuit from n-side to p-side allows the flow of electrons when the PV cell is connected to an electrical load. Electrical power is the product of the voltage multiplied by the current generated as the electrons and holes move through an external load and eventually recombine. Solar cells generate a specific amount of power and cells are tiled into modules sized to deliver the desired amount of system power. Solar modules are created by connecting a number of solar cells and are then joined into panels with specific frames and connectors.
The photovoltaic (PV) market has experienced annual growth rates exceeding 30% for the last ten years. Some articles have suggested that solar cell power production world wide may exceed 10 GWp in the near future. It has been estimated that more than 90% of all photovoltaic modules are silicon wafer based. The high market growth rate in combination with the need to substantially reduce solar electricity costs has resulted in a number of serious challenges for silicon wafer production development for photovoltaics.
In general, silicon substrate based solar energy technology follows two main strategies to reduce the costs of solar electricity by use of PV solar cells. One approach is increasing the conversion efficiency of single junction devices (i.e., power output per unit area) and the other is lowering costs associated with manufacturing the solar cells. Since the effective cost reduction due to conversion efficiency is limited by fundamental thermodynamic and physical attributes, further gains depend on basic technological advances, such as aspects of the invention disclosed herein, to deliver efficient manufacturing processes.
In a typical solar cell manufacturing process, substrates are cleaned from time to time to remove oxidation and other impurities. Processes for cleaning solar substrates, like all solar manufacturing processes, are preferrably efficient, and there is a need for efficient cleaning processes that will integrate well with an efficient overall production system.